Interplay between TiO₂ Surfaces and Organic Photochromes: A DFT Study of Adsorbed Azobenzenes and Diarylethenes

Because photochromes act as light-induced molecular switches, there is considerable interest in exporting their molecular functionality in the solid state in order to develop photoresponsive materials. In that context, we have carried out a DFT investigation of a series of photochromic derivatives of trans/cis-azobenzene and open/closed-diarylethene adsorbed onto rutile (110) and anatase (101) slabs. By varying the auxochromes and the photochromic state, we examine the trends in the surface–adsorbate interplay in terms of the electronic structure, adsorption geometries, and Bader charge transfer. Using principal components analysis, we demonstrate how the dipole moment of the isolated photochrome in the direction of the anchoring group is the most important predictor of the electronic structure of the adsorbed system. A key point of this paper is to show how the energy levels of the isolated photochrome and bare slab change as a result of adsorption and how first-principles modeling helps to rationalize and predict these trends.